This application makes reference to, incorporates the same herein, and claims all benefits accruing under 35 U.S.C. xc2xa7119 from an application entitled Apparatus And Method For Manufacturing Optical Fiber Preform By MCVD earlier filed in the Korean Industrial Property Office on Jun. 25, 1998, and there duly assigned Serial No. 98-24158 by that Office.
1. Field of the Invention
The present invention relates to the fields of methods and apparatus for manufacturing an optical fiber preform by modified chemical vapor deposition (MCVD), and more particularly, to the heating methods and apparatus used in MCVD.
2. Description of the Related Art
In the manufacture of an optical fiber preform by modified chemical vapor deposition (MCVD), as the thickness of a deposition layer of a preform increases, self-collapse of a tube occurs during the deposition, so that the thickness of the tube further increases. Thus, spread flame type burners (also known as propagation type flame burners) are typically used for general deposition processes due to their efficiency for heat transfer into a tube and for deposition therein. Due to the wide hot zone of the spread flame type burner, heat transfer into a tube is good and the deposition efficiency is high. Also a low flame pressure reduces unwanted shrinkage during the deposition. However, collapse and closure processes require a high flame pressure, unlike deposition processes. However, in the case where the spread flame type burner is used for the collapse and closure processes, due to low flame pressure, a high temperature of 2,100xc2x0 C. or more for a collapse process and 2,200xc2x0 C. for a closure process must be maintained at a low moving speed of the burner so as to soften a tube. As a result, the collapse and closure processes take up a large portion of the total processing time in manufacturing a preform, so that they are obstacles to shortening of the process time.
In a conventional method, in spite of the different mechanisms of the deposition and the collapse and the closure processes, all of these processes are performed using only a spread flame type burner suitable for deposition. Using the same spread flame type burner for the deposition, the collapse and the closure processes is adequate in the case of having a thin deposition layer and a is small tube. However, as the deposition layer becomes thick and the size of tube increases, the geometrical structure of the tube is deformed and the optical characteristics thereof are deteriorated, in addition to long process time being required.
It is therefore an object of the present invention to provide an improved apparatus and method for manufacturing a preform by modified chemical vapor deposition.
It is also an object of the present invention to provide an apparatus and method for manufacturing an optical fiber preform by modified chemical vapor deposition suitable for use with thick deposition layers and large substrate tubes.
It is a further object of the invention to provide an apparatus and method which allow for a shortened process time.
It is a yet further object of the invention to improve the geometry of the collapsed substrate tube.
It is a still further object of the invention to reduce the effects of polarization mode dispersion in the produced preform.
It is a still yet further object of the invention to reduce losses due to hydroxide diffusion in the produced preform.
To achieve the above objects, the present invention provides an apparatus for manufacturing a preform, adopting a concentrated flame type burner suitable for collapse and closure processes, in addition to a spread flame type burner suitable for a deposition process. The present invention also provides a method for manufacturing a preform using the apparatus adopting a concentrated flame type burner and a spread flame type burner.
Accordingly, there is provided an apparatus for manufacturing an optical fiber preform by modified chemical vapor deposition (MCVD), comprising heating means for heating a rotating substrate tube while moving in a predetermined direction, wherein the heating means comprises: first heating means located at the front with respect to the moving direction, for heating the substrate tube with a relatively low flame pressure; and second heating means located at the rear of the first heating means with respect to the moving direction, for heating the substrate tube with a relatively high flame pressure.
Preferably, the first and second heating means have a semi-cylindrical shape surrounding half of the tube circumference. Also, the apparatus for manufacturing an optical fiber preform further may comprise a reflector having a semi-cylindrical shape to enclose the other half of the tube circumference, the reflector facing the semi-cylindrical first and second heating means for reflecting heat produced by the first and second heating means toward the substrate tube.
Also to achieve the objects of the present invention, there is provided a method for manufacturing an optical fiber preform by modified chemical vapor deposition (MCVD), using an apparatus comprising a first heating means having a relatively low flame pressure and a second heating means having a relatively high flame pressure, the first and second heating means for heating a substrate tube while moving in a predetermined direction, the method comprising the steps of: (a) introducing a source gas together with oxygen into the substrate tube, and heating the substrate tube using the first heating means to deposit soot, to thereby deposit cladding and core layers; and (b) collapsing the substrate tube by heating the substrate tube in which the cladding and core layers have been deposited, using the second heating means.
Preferably, the collapsing step (b) comprises the sub-steps of: heating a region of the deposited substrate tube to be collapsed, using the first heating means, to thereby soften the region; and heating the softened region of the deposited substrate tube to perform collapsing.